Fix ambiguous interface for creating perturbed ensembles #65
Description
To run a perturbed ensemble in ICON, one can:
a. Give values to pinit_seed/pinit_amp when calling run-ensemble command (ICON builtin perturb functionality). If pinit_seed is not equal to 0, this will trigger the functions add_random_noise_* in icon/src/drivers/mo_atmo_nonhydrostatic.f90 and makes use of pinit_amp to pertub the prognostic variables w, vn, theta_v, exner and rho. Depending on other namelist switches, additional variables may be perturbed.
b. Make several copies of a first "ref" netcdf file and perturb them via perturb command (non-icon runs without a builtin perturbation functionality like the granules need this option)
This interface is ambiguous, and should be clarified/fixed.
Extra 1: make fields necessary for creating the ensemble file directories "required"
Extra 2: add error checks in case files/directories are not created/copied